A conventional Scanning Electron Microscope and a conventional Transmission Electron Microscope accelerate an electron beam emitted from an electron gun composed of a cold-cathode field-emission type or a field-emission type electron source, to form a fine electron beam using electron lenses, and scan the electron beam over a sample using a scanning deflector, as a primary electron beam. In the case of the SEM, obtained secondary electrons or reflected electrons are detected, thereby obtaining an image. In the case of the TEM, an image of electrons transmitted through the sample is imaged. In the electron source of the electron microscopes, zirconia may be allowed to be adhered to a side surface of a needle which is obtained by sharpening a tip of a single-crystal tungsten wire rod, and an electric field may be applied to the needle tip in a heated state to emit electrons. This is called a field-emission type electron source or a Schottky electron source.
The field-emission type electron source supplies zirconium and oxygen onto a tungsten crystal plane (100) by thermal diffusion to form a region with a low work function. The heating temperature is on the order of 1600 K to 1900 K. Normally, the field-emission type electron source is used at 1700 K to 1800 K. In PTL 1, by providing a crystal plane (100) at a tip of a tungsten needle and applying a strong electric field, thermoelectrons that surpass a potential barrier and electrons transmitted by the tunnel effect can be extracted.
A basic configuration of the field-emission type electron source is such that a needle having at its tip a crystal orientation (100) of single-crystal tungsten is spot-welded and fixed to a heating tungsten hairpin, a part of which includes zirconia which is a diffusion and supply source. The diffusion and supply source is formed around the needle as viewed from the tip side of the needle. In NPL 1, it is known that zirconia takes three types of allotropes according to heating temperature, that is, monoclinic, tetragonal, and cubic structures. PTL 2 discloses formation of a diffusion and supply source using a thin film. PTL 3 discloses a method in which a solution where zirconium hydride fine particles are mixed in an organic solvent is allowed to be adhered to a side outer portion of a tungsten needle using a dropper, and is heated under vacuum and sintered, by which a diffusion and supply source is formed.